storage: locking support

[butonic]

Currently there is no locking support. The CS3 apis should allow implementing locking functionality for eg webdav or s3.

[moscicki]

Hm... I am not totally convinced by this (maybe we should explain the rationale behind some of the APIs better in the documentation). Could you please explain the use-case?

[butonic]

WebDAV spec: https://tools.ietf.org/html/rfc4918#section-6 used by eg

• ms office tries to lock files: http://sabre.io/dav/clients/msoffice/
• wopi api needs locking Introduce persistent and explicit locking of file and folders owncloud/core#31651
• oc pr introducing hard locks Introduce persistent and explicit locking of file and folders owncloud/core#31651
• box api has locking: https://developer.box.com/reference#lock-and-unlock
• s3 object locks: https://docs.aws.amazon.com/AmazonS3/latest/dev/object-lock.html
• posix fcntl description http://pubs.opengroup.org/onlinepubs/9699919799/functions/fcntl.html

Now, for network filesystems locking is non trivial:

• On the brokenness of locking in linux by Lennart Poettering: http://0pointer.de/blog/projects/locking.html
• Samba also has an opinion on locking in https://www.samba.org/samba/news/articles/low_point/tale_two_stds_os2.html

I am happy to discuss if we want to store locks in a different place than the storage. However, I think just like search, tagging, comments, trash and versions ... locking might be implemented by a storage provider.

[butonic]

The cernsbox wopiserver uses lock files to implement locks on top of cs3. A dedicated RPC should make this clearer.